1. Field of the Invention
The present invention relates to a primary radiator provided to a satellite reflecting antenna, etc. In particular, it relates to a primary radiator having a horn portion for introducing radio waves at one end of a waveguide.
2. Description of the Related Art
FIG. 3 shows a conventional primary radiator of the kind described above. This primary radiator comprises a circular waveguide 1 having a horn portion 1a at one end and an enclosing surface 1b at the other end, and a first and second probes 2, 3 inserted into the waveguide 1 through a wall thereof. The horn portion 1a forms a cone-shaped or pyramid-shaped opening, and the waveguide 1 including this horn portion 1a is integrally formed by aluminum die-casting, etc. Further, the two probes 2, 3 form a right angle, and are located one quarter of the guide wavelength away from the enclosing surface 1a of the waveguide 1.
In the primary radiator generally configured as described above, linearly polarized waves sent from a satellite are guided into the waveguide 1 by the horn portion 1a. Of the linearly polarized waves, for instance, vertically polarized waves are received through the first probe 2 and horizontally polarized waves are received through the second probe 3. Therefore, by frequency-converting received signals from the probes 2, 3 using a converting circuit (not shown) into intermediate frequency signals and outputting them, the linearly polarized waves sent from the satellite can be received.
In the above-described conventional primary radiator, as shown by a dashed line in FIG. 4, it is known that the radiation pattern becomes a shape including a side lobe. This is because the side lobe is produced by a surface current flowing on the surface of the horn portion. For instance, when the design angle of radiation of the horn portion is 90xc2x0 (xc2x145xc2x0 with respect to the center), high side lobes are produced at around xc2x150xc2x0. Accordingly, the gain of the main lobe at the center of the angle of radiation is decreased, which brings about the problem of being unable to receive radio waves from the satellite efficiently.
According to the present invention, at least a pair of cutout portions are provided at an opening end of a horn portion to reduce a side lobe. Provision of such cutout portions causes a phase reversal of surface currents flowing through cutout portions and an adjacent projecting portion and further a considerable reduction of the side lobe, which in turn can increase the gain of a main lobe that much.
The primary radiator of the present invention comprises a waveguide having a horn portion at one end for introducing radio waves and a probe for receiving at least one wave polarization component entering the waveguide, wherein a pair of cutout portions having a depth of about one quarter of the wavelength are provided at an opening end of the horn portion, the pair of cutout portions being disposed symmetrically with respect to an axis of the waveguide.
With such a configuration, the phase reversal of the surface currents flowing through the cutout portions and the adjacent projecting portion takes place and the side lobe is reduced considerably, which in turn can increase the gain of the main lobe to achieve efficient reception of radio waves from a satellite.
In the above configuration, at least a pair of cutout portions may be provided. However, it is preferable to provide two or more pairs of cutout portions along the rim of the horn portion. Further, it is preferable to dispose at least a pair of cutout portions along the direction in which the probe extends.